1. Technical Field
Embodiments of this disclosure generally relate to an image forming apparatus, and more particularly, to an image forming apparatus for forming a multicolor image.
2. Related Art
Demand for higher-quality images is increasing in association with recent improvements in image forming apparatuses. One approach to obtaining higher-quality images involves providing electrophotographic image forming apparatuses incorporating toner of five or more colors including the usual four colors, namely, yellow (Y), magenta (M), cyan (C), and black (K). For example, JP-2007-171498-A and JP-2007-316313-A propose an image forming apparatus incorporating toner of six colors.
Such an image forming apparatus incorporating toner of five or more colors typically incorporates toner of a light color (e.g., light cyan or light yellow) and/or high-transparent toner (e.g., transparent toner) in addition to toner of the four fundamental colors, namely, yellow, magenta, cyan, and black. Such an additional color is called “auxiliary color” and is used to obtain an image with higher quality, glossiness, and color reproducibility.
The light-color toner is used to reduce the granularity of an output image, thereby enhancing image quality. The high-transparent toner is used to enhance glossiness. In some cases, a color that is difficult to reproduce by mixing yellow, magenta, and cyan may be used as an auxiliary color, or may be formed as a special color to be used in, e.g., a printer.
Image forming apparatuses typically employ a tandem method with an intermediate transfer belt to form color images. In such tandem-type image forming apparatuses, image carriers for different colors of toner are arrayed in series, each being associated with, e.g., a developing device loaded with developer having individual spectral characteristics. The tandem-type image forming apparatuses can form a color image at almost the same speed as the monochrome image forming apparatuses.
Such a tandem-type image forming apparatus includes optical systems having identical configurations based on the optical system for black. Hence, if a typical tandem-type image forming apparatus uses toner of five colors, instead of four colors, it needs 25% more space to incorporate an imaging unit and an optical scanning device for an additional color.
To minimize the additional space, components of imaging units, such as photoconductive drums, developing devices, and cleaners, may be downsized or shapes thereof may be changed to locate the imaging units closer to each other. However, downsizing the optical scanning devices is not easy while keeping a predetermined optical path length.
Hence, to downsize an optical scanning device for an auxiliary color without changing the optical path length, reflecting mirrors may be provided in the optical system between a polygon mirror serving as a deflector and a photoconductive drum to increase the number of turns in the optical path. However, such a configuration decreases light utilization efficiency of the optical system between a light source and the polygon mirror depending on the reflectance of the mirrors. In addition, the arrangement of the mirrors may change the arrangement of other optical elements and a layout of light beams. Consequently, initial characteristics and temperature characteristics of a scanning line of the auxiliary color may differ from those of the four fundamental colors over time, and particularly by variation of characteristics due to temperature changes. As a result, the auxiliary color may be noticeably misaligned or shifted from the correct position.
In such a situation, with a temperature difference among a plurality of optical scanning devices, the image forming apparatuses frequently perform a color shift correction to form a high-quality image. The color shift correction and the imaging operation are not performed simultaneously, and accordingly, productivity decreases when the color shift correction is performed frequently. As a result, a standby time lengthens, significantly degrading usability.